I have a 454, Block #14015445.  My question is, what pistons are in this motor?  Flat?  Dished?  The block has been freshened up with rings, bearings and so on.  My Heads are large oval port #336781.....73-85...oval...OPEN...454, 118cc chamber, 256/114cc ports.
I have a Comp 280 Mag cam, w/roller tip rockers and an Air Gap manifold.  I have had the motor for 1 ½ years as a spare.  My concern now is if the compression is too low, I’ll lose too much HP.  The cam needs 9:1.  Does anyone know what pistons are stock in this 2 bolt main block?
Can anyone guess the HP?  I do not want to end up (after the effort of changing engines) to end up with a 350hp big block.  450 would be ok.  Help if you can.  

The block casting# will not tell you anything about the internals. Could have flat or dished. Every 781 head I ever measured was around 122cc's, in spite of what the books may tell you. Neither flat or dished, will not give you the 9.1:1 you are looking for. Furthermore that camshaft is to long for a 9.1:1 motor. Comp. list's 9:1.1 as an absolute minimum requirement. With flat or dished pistons, composition head gaskets, -.020 deck height, and those heads, your comp. ratio is at the most, 8.2:1. It will be low and mid range torque that really suffers. With what you have now, you would be lucky to make 350HP. Without spending a lot of money on parts, and doing some machine work, the best I can recommend is changing the cam for something more suitable and going with a .020 steel shim head gasket to get the quench clearance correct (.038) and squeek comp. up a hair.